1. Field of the Invention
The present invention relates to a device for determining misfiring in cylinders of a multi-cylinder engine.
2. Description of the Related Art
In a multi-cylinder engine, misfiring cylinders can be determined accurately by detecting the combustion pressure or exhaust gas temperature of each cylinder of the engine. However, to detect misfiring cylinders by this method, it is necessary to install a combustion pressure sensor or exhaust gas temperature sensor on each cylinder of the engine. Since many sensors are involved, the initial cost of the device becomes higher, and the reliability of the device becomes lower due to the possibility of failure of the sensors. Therefore, in prior arts, an effort is directed to provide a device for determining misfiring cylinders in a multi-cylinder engine which is reliable and less expensive, by reducing the number of the sensors involved.
For example, Japanese Unexamined Utility Model Publication (Kokai) No. 60-8429 discloses a device for determining misfiring cylinders in a multi-cylinder engine based on the difference in the torsional vibration of the output shaft of the engine peculiar to combinations of the misfiring cylinders. This device comprises a torsional pulse generator which generates pulse signals representing a rotational angle of the output shaft at a predetermined longitudinal position thereof, and a reference pulse generator which generates a pulse signal every time the piston of a predetermined cylinder reaches the top dead center, and a rotation pulse generator which generates a predetermined number of pulse signals dividing the period between said reference pulse signals at regular intervals. The device measures the time lag between the torsional pulse and the rotation pulse. The amount of this time lag corresponds to instantaneous torsional angle of the output shaft of the engine i.e., the instantaneous amplitude of the torsional vibration of the output shaft. Then this time lag signal is frequency-analyzed and a spectrum thereof is obtained. The spectrum pattern obtained is then compared with the patterns previously obtained by cutting the fuel supply to the various cylinders of the engine, and the misfiring cylinders are determined based on the similarities of the patterns.
By the device described above, the misfiring cylinders can be determined without using the combustion pressure sensors nor the exhaust gas temperature sensors. However, in the above device, there is a problem that the accuracy of the determination of the misfiring cylinders is relatively low. Since the device utilizes the time lag between the torsional pulse and the rotation pulse to determine the misfiring cylinders and this time lag is affected not only by the misfiring of the cylinders but also by the fluctuation of the engine rotational speed, the spectrum pattern obtained is also affected by the fluctuation of the engine rotational speed. Accordingly, the accuracy of the determination itself is also affected by the fluctuation of the engine rotational speed.